Accelerated thermal aging of petroleum-based ferrofluids

The effect of elevated temperature on the physical and insulating propertie s of ferrofluid specifically developed for use as a liquid dielectric (D-fl uid) for poser transformers has been investigated. The D-fluid was produced as a colloidal mix of a specifically synthesized ferrofluid with a convent ional mineral oil, and it was subjected to thermal aging conditions modeled after a typical power transformer where the insulation fluid is expected t o retain its dielectric performance for about 40 years of continuous servic e in a sealed tank. The well-known Arrhenius relationship was employed to m odel "life in service" for up to 40 years at 105 degrees C which correspond ed to holding the samples in sealed jars for 10 weeks at 185 degrees C. Ano ther set of small ampules (5 mi) was prepared to test the main physical pro pertius after even longer aging. D-fluid tested after a period of 34 and 50 weeks at 185 degrees C showed no degradation of thermal or colloid stabili ty. The dielectric colloid was also subjected to a 21 day-long test at 110 degrees C in a sealed jar in the presence of typical transformer materials: copper, cellulose, and silicon steel (so-called "bomb" test). Finally, the ferrofluid went through an oxidation stability test (ASTM D2440). Test res ults show that the newly developed dielectric colloid satisfies the long-te rm service requirements the transformer users typically apply to convention al mineral oils. (C) 1999 Published by Elsevier Science B.V. All rights res erved.